U.S. patent application number 10/565612 was filed with the patent office on 2007-06-14 for microlithographic projection exposure apparatus and method for introducing an immersion liquid into an immersion space.
This patent application is currently assigned to CARL ZEISS SMT AG. Invention is credited to Bernhard Gellrich, Jens Kugler, Gerd Reisinger, Dieter Schmerek.
Application Number | 20070132969 10/565612 |
Document ID | / |
Family ID | 34129463 |
Filed Date | 2007-06-14 |
United States Patent
Application |
20070132969 |
Kind Code |
A1 |
Gellrich; Bernhard ; et
al. |
June 14, 2007 |
Microlithographic projection exposure apparatus and method for
introducing an immersion liquid into an immersion space
Abstract
The invention relates to a projection exposure system for
microlithography, said system comprising an illumination device for
generating a projection light, and a projection objective
comprising a plurality of optical elements such as lenses (L5) and
enabling a reticle that can be arranged in an object plane of the
projection objective to be imaged onto a light-sensitive surface
(26) that can be arranged in an image plane of the projection
objective and is applied to a carrier (30). The inventive system is
also provided with an immersion device between an image-side last
optical element (L5) of the projection objective and the
light-sensitive surface (26), for introducing an immersion liquid
(34) into an immersion chamber (50). Said immersion device
comprises means (44; 66) which can prevent the appearance of gas
bubbles (48) in the immersion liquid (34), affecting the imaging
quality, and/or can remove existing gas bubbles (48). Said means
can be, for example, an ultrasound source (66) or a degasifier
(44).
Inventors: |
Gellrich; Bernhard; (Aalen,
DE) ; Reisinger; Gerd; (Oberkochen, DE) ;
Schmerek; Dieter; (Huettlingen, DE) ; Kugler;
Jens; (Heubach, DE) |
Correspondence
Address: |
FACTOR & LAKE, LTD
1327 W. WASHINGTON BLVD.
SUITE 5G/H
CHICAGO
IL
60607
US
|
Assignee: |
CARL ZEISS SMT AG
Carl-Zeiss-Str. 22,
Oberkochen
DE
73447
|
Family ID: |
34129463 |
Appl. No.: |
10/565612 |
Filed: |
July 8, 2004 |
PCT Filed: |
July 8, 2004 |
PCT NO: |
PCT/EP04/07456 |
371 Date: |
July 6, 2006 |
Current U.S.
Class: |
355/53 ;
355/30 |
Current CPC
Class: |
G03F 7/70341
20130101 |
Class at
Publication: |
355/053 ;
355/030 |
International
Class: |
G03B 27/42 20060101
G03B027/42 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 2003 |
DE |
103 33 644.3 |
Claims
1-16. (canceled)
17. A projection exposure apparatus for microlithography,
comprising: an illumination system for generating projection light;
a projection objective comprising a plurality of optical elements
for imaging a reticle onto a photosensitive surface; and, an
immersion device for introducing an immersion liquid into an
immersion space formed between a last optical element on the image
side of the projection objective and the photosensitive surface,
wherein said immersion device comprises a suction device having a
suction nozzle opening into the immersion space and is configured
to extract gas bubbles from the immersion liquid during the
exposure operation.
18. The apparatus of claim 17, wherein the suction device is
configured to extract the gas bubbles during operation of the
apparatus.
19. The apparatus of claim 17, wherein a support for the
photosensitive surface is configured to be displaceable in a
scanning direction of the projection exposure apparatus, and
wherein the immersion space is bound by side walls only parallel to
the scanning direction, but not perpendicular thereto.
20. A projection exposure apparatus for microlithography,
comprising: an illumination system for generating projection light;
a projection objective comprising a plurality of optical elements
for imaging a reticle onto a photosensitive surface; an immersion
space formed between a last optical element on the image side of
the projection objective and the photosensitive surface, said
immersion space being confined by at least one side wall; and, an
ultrasound source which induces oscillations in said at least one
side wall for removing gas bubbles in an immersion liquid
introduced into the immersion space.
21. A projection exposure apparatus for microlithography,
comprising: an illumination system for generating projection light;
a projection objective comprising a plurality of optical elements
for imaging a reticle onto a photosensitive surface; and, an
immersion device for introducing an immersion liquid into an
immersion space formed between a last optical element on the image
side of the projection objective and the photosensitive surface,
wherein said immersion device comprises circulation means for
circulating the immersion liquid in the immersion space, said
circulation means comprising a circulating pump, a filling nozzle
opening into the immersion space, a suction nozzle opening into the
immersion space, and a degasser for removing gas bubbles from the
immersion liquid.
22. The apparatus of claim 21, wherein the degasser comprises: a
run-off surface that is obliquely arranged so that immersion liquid
which is applied from above runs down the surface; and, means for
establishing a negative pressure above the run-off surface.
23. The apparatus according to claims 21, wherein the circulation
means are integrated into the projection objective.
24. A projection exposure apparatus for microlithography,
comprising: an illumination system for generating projection light;
a projection objective comprising a plurality of optical elements
for imaging a reticle onto a photosensitive surface; and, an
immersion device for introducing an immersion liquid into an
immersion space formed between a last optical element on the image
side of the projection objective and the photosensitive surface,
wherein said immersion device is configured to introduce a flushing
liquid different from the immersion liquid into the immersion
space.
25. A method for introducing an immersion liquid into an immersion
space which is formed between a last optical element on the image
side of a projection objective of a projection exposure apparatus
for microlithography and a photosensitive surface to be exposed,
comprising the following steps: wetting the photosensitive surface
and the last optical element with the immersion liquid, wherein a
support for the photosensitive surface is outside a beam path of
the projection exposure apparatus; bringing the support up to the
last optical element in a movement parallel to an image plane of
the projection objective, so that the immersion liquids on the last
optical element and on the photosensitive surface touch; and,
introducing the support completely into the beam path in a movement
parallel to the image plane until the support reaches the required
position for exposure.
26. A projection exposure apparatus for microlithography,
comprising: an illumination system for generating projection light;
a projection objective, which has an image plane and comprises a
plurality of optical elements for imaging a reticle onto a
photosensitive surface arranged in the image plane; and, a
wedge-shaped immersion space formed between a last optical element
on the image side of the projection objective and the
photosensitive surface.
Description
[0001] The invention relates to a projection exposure apparatus for
microlithography, having an illumination device for generating
projection light, a projection objective with a plurality of
optical elements, by which a reticle that can be arranged in an
object plane of the projection objective can be imaged onto a
photosensitive surface, which can be arranged in an image plane of
the projection objective and is applied on a support, and having an
immersion device for introducing an immersion liquid into an
immersion space between a last optical element on the image side of
the projection objective and the photosensitive surface. The
invention also relates to a method for introducing an immersion
liquid into such an immersion space.
[0002] A projection exposure apparatus and a method of this type
are known from EP 0 023 243 A1. In order to hold a semiconductor
wafer to be exposed, this known projection exposure apparatus has
an open-topped container whose upper edge is higher than the lower
delimiting surface of the last lens on the image side of the
projection objective. Feed and discharge lines for an immersion
liquid open into the container, and these are connected to a pump,
a temperature regulating device and a filter for cleaning the
immersion liquid. When the projection exposure apparatus is in
operation, the immersion liquid is circulated in a liquid circuit
while an intermediate space, which is left between the lower
delimiting surface of the last lens on the image side of the
projection objective and the semiconductor wafer to be exposed,
remains filled. The resolving power of the projection objective is
intended to be increased because of the higher refractive index of
the immersion liquid, which in this known projection exposure
apparatus preferably corresponds to the refractive index of the
photosensitive layer applied on the semiconductor wafer.
[0003] A projection exposure apparatus having an immersion device
is furthermore known from WO 99/49504. In this projection exposure
apparatus, the feed and discharge lines for the immersion liquid
open directly at the lower delimiting surface of the last lens on
the image side of the projection objective. Using a plurality of
such feed and discharge lines, which may for example be arranged in
a ring around the last lens on the image side, makes it possible in
particular to obviate a surrounding container since immersion
liquid flowing away laterally is sucked out and delivered so that
the immersion space between the last lens on the image side and the
photosensitive surface remains filled with immersion liquid.
[0004] Generally speaking, immersion lithography promises very
large numerical apertures and a greater depth of focus. However,
the imaging quality of microlithographic immersion objectives
leaves something to be desired in many cases.
[0005] It is therefore an object of the invention to provide a
projection objective of the type mentioned in the introduction,
with which it is possible to achieve a higher imaging quality.
[0006] This object is achieved in that the immersion device
comprises means by which the creation of gas bubbles in the
immersion liquid can be prevented and/or gas bubbles which have
already been created can be removed.
[0007] The invention is based on the discovery that bubbles in the
immersion liquid are one of the causes of imaging errors. This is
because the immersion liquids used, for example water or particular
oils, contain inherently dissolved gases which enter the gas phase
in the event of pressure and/or temperature changes and thereby
lead to the creation of bubbles.
[0008] Such pressure changes occur, for example, when the immersion
space between the last optical element on the image side and the
photosensitive surface is filled with the immersion liquid before
the start of projection. It is furthermore always necessary to fill
the immersion space with immersion liquid when a support having an
already exposed photosensitive layer is replaced by a support whose
photosensitive layer is still unexposed.
[0009] Movements of the support relative to the projection
objective, such as those which occur both in pure steppers or
scanners and in projection exposure apparatus for which step-wise
and continuous movements of the support are combined, are another
cause of pressure variations which lead to the creation of bubbles.
Particularly at the edges of the photosensitive surface, undesired
pressure variations can occur during these movements. Pressure
variations that lead to bubble formation can furthermore occur in
the intermediate regions of particular surface structures.
[0010] A similar problem is also encountered with measurement heads
for projection objectives, which are introduced into the image
plane instead of the support in order to determine the imaging
quality of the projection objective. The sensor head is moved
through under the projection objective within the image plane
during the measurements, so that bubble formation may likewise take
place.
[0011] The immersion device according to the invention may, for
example, comprise a suction device for extracting gas bubbles,
which has a suction gland opening into the immersion space. This
suction gland, which can be provided in addition to a suction gland
that may furthermore be required in order to circulate the
immersion liquid, preferably extracts immersion liquid, with
bubbles contained in it, in the immediate vicinity of the last
optical element on the image side, so that these bubbles cannot
impair the imaging quality.
[0012] If the support can be displaced in a scanning direction of
the projection exposure apparatus, then it is expedient for the
immersion device to have a side wall which at least partially
bounds the immersion space and is designed so as to substantially
prevent at least lateral run-off of the immersion liquid
transversely to the scanning direction. This reduces
inhomogeneities of the immersion liquid perpendicularly to the
scanning direction. Inhomogeneities parallel to the scanning
direction, on the other hand, are less critical when scanning
because averaging is carried out in this direction by the
scanning.
[0013] It is nevertheless particularly preferable for the side wall
to completely, preferably annularly, enclose the last optical
element on the image side. This prevents any undesired run-off of
immersion liquid.
[0014] Another way of removing bubbles which have been created in
the immersion liquid is for an ultrasound source, by which the side
wall can be set in oscillation, to be coupled to the side wall.
Since the bubbles per se do in fact break up by themselves but the
time taken for this is relatively long, by applying an ultrasound
field acting on the side wall it is possible to excite the
immersion liquid in oscillations so that break-up of the bubbles
can be significantly accelerated. This is because the bubbles are
set into high-frequency oscillations and thus deformed by the
ultrasound field, so that the break-up process is accelerated.
[0015] It is furthermore preferable for the immersion device to
have circulation means for circulating the immersion liquid in the
immersion space, which comprise a circulating pump, a filling gland
opening into the immersion space and a suction gland opening into
the immersion space. By means of this, in circulating operation, it
is possible for the immersion liquid to be constantly cleaned,
thermally regulated and also degassed, if a degasser for removing
gas bubbles from the immersion liquid is additionally provided.
[0016] A degasser suitable for this may, for example, have a
preferably frustoconical run-off surface arranged in an inclined
fashion, onto which immersion liquid can be applied from above and
over which a negative pressure can be set up. The effect of this
negative pressure is that gases, which are dissolved in the liquid
film distributed over the run-off surface, enter the gas phase and
emerge from the film.
[0017] If the support can be displaced in a scanning direction of
the projection exposure apparatus, then it is furthermore
preferable for the support to be arranged with respect to the
projection objective so as to reduce the extent of the immersion
space perpendicularly to the image plane along the scanning
direction. Since generally both the photosensitive surface and the
image-side delimiting surface of the last optical element on the
image side are plane, this arrangement leads to an essentially
wedge-shaped immersion space which converges acutely towards the
scanning direction. This wedge-shaped immersion space leads to a
suction effect during the scanning movement of the support, so that
circulation of the immersion liquid in the immersion space requires
only a low pump power. Another advantage of the wedge-shaped
geometry of the immersion space is that a more uniform fluid flow
is created overall in the immersion space.
[0018] In this context, it is naturally preferable for the suction
gland of the circulation means to be arranged before the filling
gland of the circulation means in the scanning direction, since in
this way extraction of the immersion liquid is assisted by the
scanning movement.
[0019] In a preferred configuration of the invention, the
circulation means are integrated into the projection objective,
preferably in a frame of the last optical element on the image
side. It is even feasible to integrate the circulation means into
the optical element itself. These measures contribute to keeping
the immersion space as smooth and edge-free as possible, and
thereby to avoiding turbulence of the immersion liquid which could
lead to the creation of bubbles.
[0020] Another way in which the occurrence of bubble formation can
itself be prevented is for the photosensitive surface to be held in
a closed cassette completely filled with immersion liquid, in the
object-side wall of which the last optical element on the image
side of the projection objective is held so that it can be
displaced in a direction parallel to the image plane. In this way,
the immersion liquid can be hermetically isolated from the
surroundings, so that the other parts of the projection exposure
apparatus cannot be contaminated by the immersion liquid. Such a
cassette can furthermore be used in a vacuum.
[0021] Since it is possible both to introduce the support into the
cassette and fill the latter with the immersion liquid outside the
beam path of the projection exposure apparatus, these measures can
be carried out without time constraint, so that the ingress of gas
bubbles can be reliably prevented with the aid of suitable
measures. It is furthermore possible to clean the cassette and
remove used immersion liquid outside the beam path, and therefore
without time constraint.
[0022] In order to prevent the creation of gas bubbles owing to the
displacement of the last optical element on the image side, the
cassette may be in communication with a reservoir using which
immersion liquid can optionally be topped up or to which excess
immersion liquid can be discharged.
[0023] It is, however, preferable for the object-side wall of the
cassette to be designed so that the volume filled with the
immersion liquid in the cassette does not change when the last
optical element on the image side is displaced. In this way, at no
time during operation does the immersion liquid come in contact
with the surroundings and, in particular, in contact with gases as
would be the case with an additional reservoir.
[0024] Such a wall may, for example, be produced using a bellows or
an arrangement of plate-shaped sub-elements, which can be slid over
or into one another in the displacement direction of the last
optical element on the image side.
[0025] It is furthermore particularly preferable that a flushing
liquid different from the immersion liquid can be introduced into
the immersion space by the immersion device. Residues of used and
contaminated immersion liquid can be removed from the immersion
space with the aid of the flushing liquid.
[0026] In order to assist cleaning, the support with the
photosensitive surface may be replaceable by a cleaning plate,
which can be set in motion within a plane parallel to the image
plane.
[0027] Even the way in which the immersion liquid is introduced
into the immersion space for the first time has an influence on the
creation of bubbles. The invention therefore also relates to a
method for introducing an immersion liquid into an immersion space
which is formed between a last optical element on the image side of
a projection objective of a projection exposure apparatus for
microlithography and a photosensitive surface to be exposed, which
is applied on a support.
[0028] In order to minimise the formation of bubbles during this
process, the following steps are provided: [0029] a) wetting the
photosensitive surface and the last optical element on the image
side with immersion liquid, the support being outside the beam path
of the projection exposure apparatus; [0030] b) bringing the
support up to the last optical element on the image side in a
movement parallel to the image plane, so that the immersion liquids
lying on the last optical element on the image side and on the
photosensitive surface touch; [0031] c) introducing the support
completely into the optical path in a movement parallel to the
image plane, until the support reaches the required position for
exposure.
[0032] Other advantages and features of the invention will be found
in the following description with reference to the drawings, in
which:
[0033] FIG. 1 shows a meridian section through a projection
exposure apparatus according to the invention in a highly
simplified schematic representation which is not true to scale;
[0034] FIG. 2 shows an immersion device according to another
exemplary embodiment with a degasser;
[0035] FIG. 3 shows the degasser indicated in FIG. 2 in a sectional
representation;
[0036] FIG. 4 shows a detail of an immersion device according to a
further exemplary embodiment of the invention;
[0037] FIG. 5 shows a cassette with a support held in it, and a
last lens on the image side held so that it can be displaced.
[0038] FIG. 1 shows a meridian section through a microlithographic
projection exposure apparatus, denoted overall by 10, in a highly
simplified schematic representation. The projection exposure
apparatus 10 has an illumination device 12 for generating
projection light 13, which comprises inter alia a light source 14,
illumination optics indicated by 16 and a diaphragm 18. In the
exemplary embodiment represented, the projection light has a
wavelength of 157 nm.
[0039] The projection exposure apparatus 10 furthermore has a
projection objective 20 which contains a multiplicity of lenses,
only some of which (denoted by L1 to L5) are represented by way of
example in FIG. 1 for the sake of clarity. Owing to the short
wavelength of the projection light 13, the lenses L1 to L5 are made
of calcium fluoride crystals which are still sufficiently
transparent even at these wavelengths. The projection objective 20
is used to project a reduced image of a reticle 24, arranged in an
object plane 22 of the projection objective 20, onto a
photosensitive surface 26 which is arranged in an image plane 28 of
the projection objective 20 and is applied on a support 30.
[0040] The support 30 is fastened on the bottom of an open-topped
container 32 in the shape of a trough, which can be displaced (in a
way which is not represented in detail) parallel to the image plane
28 with the aid of a displacement device. The container 32 is
filled sufficiently with an immersion liquid 34 so that, during
operation of the projection exposure apparatus 10, the projection
objective 20 is immersed with its last lens L5 on the image side in
the immersion liquid 34. This lens L5 is a comparatively thick lens
having a high aperture in the exemplary embodiment represented,
although the term "lens" is in this context also intended to
include a plane-parallel plate.
[0041] Via a feed line 36 and a discharge line 38, the container 32
is connected to a treatment unit 40 which (in a manner known per se
and therefore not represented in detail) contains a circulating
pump, a filter for cleaning immersion liquid 34 and a temperature
regulating device. The treatment unit 40, the feed line 36, the
discharge line 38 and the container 32 together form an immersion
device denoted by 42, in which the immersion liquid 34 is
circulated while being cleaned and kept at a constant temperature.
The immersion device 32 is used in a manner known per se to
increase the resolving power of the projection objective 20.
[0042] The treatment unit 40 furthermore contains a degasser
indicated by 44, the structure of which will be explained in more
detail below with reference to FIG. 3. Gaseous constituents, which
could enter the gas phase in the container 32 and thereby lead to
the formation of bubbles, are drawn from the circulating immersion
liquid 34 by the degasser 44.
[0043] FIG. 2 shows another exemplary embodiment of an immersion
device in an enlarged detail of the image-side end of the
projection objective, parts corresponding to one another in FIGS. 1
and 2 being provided with the same reference numerals. It can be
seen particularly clearly in this enlarged representation that--as
in the exemplary embodiment shown in FIG. 1--the last lens L5 on
the image side is held in a frame so that the plane image-side
delimiting surface of the lens L5 merges into the frame 46 without
forming projections or gaps. This reduces the likelihood that
turbulence may form in this transition region, and consequently
that bubbles 48 may be created.
[0044] The volume lying in the beam path of the projection
objective 20 between the lens L5 and the photosensitive surface 26
is filled with immersion liquid 34, and will therefore be referred
to below as an immersion space 50. The immersion space 50 is sealed
laterally by an open-topped ring 52, and towards the photosensitive
surface 26 by a sealing element 54. The sealing element 54 may be
obviated if the pressure of the surrounding gas is high enough to
prevent the immersion liquid 34 from emerging. The ring 52 contains
a first bore 56, which is connected to the feed line 36 and whose
end opening into the immersion space 50 forms a filling gland 58.
The ring 52 furthermore contains a second bore 60, which is
connected to the discharge line 38 and whose end opening into the
immersion space forms a suction gland 62. The feed line 36 and the
discharge line 38 are connected to a circulating pump 64, which can
circulate the immersion liquid 34 in a closed circuit.
[0045] Upstream of the circulating pump 64 in the feed line 36,
there is a degasser 44 which sets up a large negative pressure over
a thin liquid film, and thereby draws gases dissolved in the
immersion liquid 34 therefrom and greatly undersaturates it. Owing
to this undersaturation, gases still dissolved in the immersion
liquid 34 remain for the very dominant part in solution even when
pressure or temperature variations take place.
[0046] Particularly when filling the immersion space 50 or when
moving the support 30 relative to the last lens L5 on the image
side, the pressure and temperature variations may nevertheless be
so great that bubbles 48 can be created. In order to break up
bubbles 48 which have already been created, an ultrasound source 66
is additionally provided which can act on the ring 52, as indicated
by a double arrow in FIG. 2. The bubbles 48 are therefore set in
high-frequency motion and thereby deformed, so that the bubbles 48
break up rapidly.
[0047] FIG. 3 schematically shows the degasser 44 in a cross
section. Immersion liquid 34 is pumped into an annular distributor
line 70 by means of a pump 68 via the discharge line 60 in the
direction indicated by arrows.
[0048] From the distributor line 70, the immersion liquid 34 flows
out as a thin film 72 down a run-off surface 74, frustoconically
designed in the exemplary embodiment represented, which is
preferably arranged in an inclined fashion, and finally collects in
an outflow line 76, which is connected to the feed line 36 via the
pump 64. The space 78 remaining over the run-off surface 74 is in
communication with a vacuum pump 82 via a suction line 80, and can
thereby be evacuated. The effect of the negative pressure thus
created in the space 78 is that gases dissolved in the immersion
liquid 34 are drawn from it.
[0049] FIG. 4 shows a part of an immersion device according to
another exemplary embodiment, in which the immersion space 50 is
framed by side walls only laterally, i.e. parallel to the plane of
the paper, but not transversely to a scanning direction indicated
by an arrow 84. The scanning direction 84 is the direction in which
the support 30 moves under the lens L5 during the scanning
operation. This relative motion between the support 30 and the lens
L5 creates a transport effect, by which immersion liquid 34
emerging from a filling gland 58' opening into the immersion space
50 is delivered to a suction gland 62', which likewise protrudes
into the immersion space 50. This transport motion prevents
immersion liquid 34 escaping from the immersion space 50 counter to
the scanning direction 84.
[0050] The transport effect can additionally be amplified if the
distance indicated by d in FIG. 4, between the lens L5 and the
photosensitive surface 26, decreases continuously in the scanning
direction. The immersion space 50 can then have a wedge-shaped
configuration which amplifies the transport effect and leads to
particularly uniform filling of the immersion space 50 with
immersion liquid 34. In order to produce such a wedge-shaped
immersion space 50, for example, the support 30 with the
photosensitive surface 26 applied on it may be slightly tilted. In
order to achieve a correspondingly tilted image plane, the
projection objective 20 may for example contain a wedge-shaped
correcting element.
[0051] The frame 46' of the lens L5 also includes a suction gland
86, the purpose of which is to immediately extract gas bubbles
created in the exit region of the filling gland 58', before they
can reach the image-side delimiting surface of the lens L5 and
cause imaging errors there.
[0052] FIG. 5 shows a further way in which it is possible to
prevent the creation of bubbles in the immersion liquid 34. In this
exemplary embodiment, the support 30 with the photosensitive
surface 26 applied on it is held entirely in a cassette 90 closed
all around, the entire remaining volume of which is filled with the
immersion liquid 34. A last lens L5' on the image side is fitted
into the object-side wall, designed as a bellows 92, so that the
lens L5' can be displaced in the scanning direction indicated by an
arrow 84', but without the volume inside the cassette 90 thereby
changing. This ensures that the immersion liquid 34 in the cassette
90 cannot enter in contact with a gas at any time.
[0053] A separate apparatus is preferably provided in order to
introduce the support 30 with the photosensitive surface 26 into
the cassette 90, and fill the remaining volume with the immersion
liquid 34. This apparatus may comprise a vacuum pump, with which it
is possible to ensure that the immersion liquid substantially freed
of dissolved gases in the degasser can be introduced into the
cassette 90, but without entering in contact with a gas. Even if
the immersion liquid 34 in the cassette 90 is set in motion when
the lens L5' is displaced during the scanning process, in this way
virtually no gases can enter the gas phase and thereby give rise to
bubbles.
* * * * *